Strange coming from wordy old me, but less is more. If it's a paragraph, people are just going to ignore it.
Something like:
NOTE: If your question is about something best explained visually, post a screenshot. Here's how: [LINK]
I fully sympathize with the general idea. Less is more.
Yet, that brief wording might (it surely will by some) be read as "hey, that's great, now I won't need to use any words at all".
Secondly, my urging to use the 'Preview' button comes from seeing quite a few posts with 'broken image' icons, or screenshots at an enormous scale.
Using 'Preview' then will confront a poster that he should make some improvements before sharing his composed message to the world.